Heat treatment to dissolve low melting phases in superalloys

ABSTRACT

A progressive heat treatment of a superalloy is disclosed wherein the segregated low melting phases are dissolved in solid solution and diffused through the structure without melting the matrix metal and without incipiently melting any of the low melting phases. The heat treatment begins at a temperature just below that of the lowest melting constituent and finishes at a temperature below the fusion temperature of the alloy. The time of heat treatment at each successive increasing temperature is sufficient to accomplish enough diffusion to avoid melting on subsequent temperature increases, until the various phases are substantially solutionized.

[ Aug. 21, 1973 United States Patent [191 Walker et al.

[5 HEAT TREATMENT T0 DISSOLVE LOW 3,272,666 9/1966 Symonds............................... 148/13 MELTING PHASES IN SUPERALLOYS [75] Inventors: James L. Walker, Schenectady; Primary Examiner Richard Dean Thomas Sawyer Baum) Lake Attorney-Richard R. Brainard et a1. both of NY.

[73] Assignee: General Electric Company,

Schenectady, NY.

Aug. 2, 1972 [22] Filed:

2 APP] 277,373 A progressive heat treatment of a superalloy is disclosed wherein the segregated low melting phases are Related ll Dam dissolved in solid solution and diffused through the [63] Continuation of Ser. No. 100,053, Dec. 21 structure without melting the matrix metal and without abandoned.

incipiently melting any of the low melting phases. The heat treatment begins at a temperature just below that 20 1 H6137 l/0..l 8.... 5 lM ul l C 5 7 n 1 an n 700 WM ,4 4 B ml 1B n MB m u m "e U M I C 0 & LM 0 I] U hF 1] 2 8 5 55 [rt sion to avoid melting on subsequent temperature increases, until the various phases are substantially solutionized.

4 Claims, 5 Drawing Figures 3,310,440 Piearcey 148/13 PATENTEUAUGZI 1915 37531790 SHEEI 1 or 3 [r2 ve r7 tors: (75m es L. We lker; Thomas F? 3d w er; m

Their Attorney.

PATENTED AUG 21 I975 sum 2 or 3 [r7 vent'or-s: m s L. We: er-;

A 't'or-ney.

Thomas F So? wyer b y The/r PATENTED AUS 2 1 I975 SHEET 3 OF 3 R. 5. .1 nw mn I .m.

Thomas FT Sc? wyer b y The [r7 ve r7 or-s Jmes .L. Wd/Ker Attorney.

HEAT TREATMENT T DISSOLVE LOW MELTING PHASES IN SUPERALLOYS This is a. continuation, of application Ser. No. 100,053, filed Dec. 2l, 1970, now abandoned.

This invention relates to superalloys and more particularly to the heat treatment of those alloys having segregated low melting phases in the as-cast condition, in order to improve their properties.

BACKGROUND In nickel base superalloys, of which Rene 100 and Rene 80 are typical examples, non-equilibrium phases are produced as a result of segregation during solidification. Segregates tend to form because of the nonequilibrium conditions which occur during the normal cooling of the alloy as it solidifies.

Heretofore, the alloy Rene 100 was considered as being non-solutionable. Whenever this alloy was heated above the solvus temperature in an attempt to put the different microscopic phases into solid solution, the lowest melting phase would melt. This lead to the conclusion, prevalent in the art, that within practical time limits the differentlow melting phases of these alloys could not be put into solid solution.

In order to improve the physical properties of an alloy, in general it is known that if the various constituents can be placed into solid solution and then precipitated out of solid solution during an age hardening" heat treatment, the properties can be substantially improved. However, heretofore there has been no method (within practical time limits) by which alloys having substantial quantities of segregated low melting phases could be heat treated in order to place all their constituents into solid solution without incurring the problem of incipient melting.

SUMMARY OF THE INVENTION heat treated alloy body having improved physical properties.

In accordance with these and other objects of the invention, a method is provided wherein a specimenof the alloy to be heat treated is heated up to a predetermined temperature, removed and microscopically examined for incipient melting. If incipient melting has occurred, the predetermined temperature is decreased.

If incipient melting had not occurred, the predetermined temperature is increased. A starting temperature for solution heat treatment is thus selected which is just below the temperature at which incipient melting of the lowest melting phase occurs. The progressive heat treatment then proceeds at increasing temperatures up to a point below the temperature at which the solid solution of the alloy melts, but proceeding at a rate sufficiently slow to accomplish enough diffusion to avoid melting .the low melting phases on subsequent temperature increases, until all the low melting phases are substantially solutionized. After cooling to room tempera ture, the-solutionizing heat treatment can thenbe followed by aprecipitation hardening heat treatment.

low melting phases go intosolid solution during the;

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understoodfrom the following description taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a photomicrograph (.150 X) of a section of a Rene 100 alloy subjected to a load of 20,000 psi at a temperature of 1,800 F;

FIG. 2 is a photomicrograph (1,000 X) of Rene 100 alloy in the as-cast condition;

FIG. 3 is a photomicrograph (1,000 X) of the alloy of FIG. 2, showing the segregated area in the incipient melted condition;

FIG. 4 is a photomicrograph (1,000 X) of the same alloy Rene 100, showing the segregate partially solutionized into the surrounding alloy; and

FIG. 5 is a photomicrograph (1,000 X) of the same alloy, Rene 100, showing the alloy after the solutionizing heat treatment and air cooling.

Table I below lists the compositions of two typical known nickel base superalloys, both of which form segregatedareas in the as-cast condition, similar to that illustratedin FIG. 2 of the drawings. Each of these alloys may be solution heat treated by the method according to the present invention.

l-leretofore, it has not been possible to fully solutionize alloys having substantial quantities of segregated low melting phases without the occurrence of incipient melting of such phases. Therefore, the full potential strength of such alloys has heretofore not been achieved. The present invention teaches how such full solutionizing and resultant enhancing of properties can be achieved.

TABLE I Nickel Base Superalloys Element gates has its own individual incipient melting tempera ture and that the incipient melting temperature is specific for each composition. The incipient melting temperatures for the lowest melting segregates of the alloys of Table I are as follows:

"C "F Rene H to I200C (2174' to 2l92F) Rene 1180' to ll90C (2l56 to 2l74F) The incipient melting temperatures increase as the heat treatment.

DISCUSSION Among the problems of the prior art, when samples. of such alloys in the as-cast condition were subjected" to stress at elevated temperatures, cracks sometimes developed. FIG. 1, for example, shows typical actual voids in the ascast Rene 100 alloy which form in the segregated zones under stress. The particular conditions of the test in Example I, were 20,000 psi load at 1,800 F. The cracks or voids developed in these segregated areas due to the fact that these areas were the weakest zones of the alloy because they contained the low melting phases.

Accordingly, one of the aims of the present invention is to solutionize the various phases which constitute the segregated weak areas which may cause the condition illustrated in FIG. 1.

Thus, according to the present invention, the different segregates present in the as-cast body can be solutionized by gradually diffusing the segregated phases into the surrounding metal at a temperature below the incipient melting temperature. Then, after the phases of the segregate are placed into solid solution, they can be precipitated out more uniformly than they had been in the as-cast condition, to further improve the properties of the alloy. This type of heat treatment has two effects: first, it allows one to make use of essentially all of the possible precipitate which can be formed in the alloy, and secondly, it helps eliminate the areas in which the cracks or voids can form under conditions of stress at high temperature.

FIG. 2 is a photomicrograph (1,000 X) of a specimen of the Rene 100 alloy in the as-cast condition, and the segregated areas of low melting constituents are clearly visible.

EXAMPLE I To determine the starting temperature for the heat treatment, a number of specimens of nickel base alloy Rene 100 were heated at successively increasing tern peratures for minutes at each temperature and microscopically examined for evidence of incipient melting. Incipient melting, as illustrated in FIG. 3, was first observed in the specimen heated at l,l80 C (2,156 F). The starting temperature for the progressive heat treatment was thus determined to be l,l70 C (2,138 F). This temperature so determined was just below the incipient melting temperature for the as-cast alloy Rene 100.

The progressive heat treatment temperature was then gradually increased, and periodically specimens were removed and examined for incipient melting to make sure that diffusion without such incipientmelting had taken place before the temperature was further increased. The temperature was then increased gradually to l,220 C. During this increase, specimens were periodically removed and examined for incipient melting and to determine the progress of the solutionizing of the low melting phases. The temperature at which microscopic examination showed the the low melting phases were substantially solutionized was l,220 C.

EXAMPLE 2 The procedure of Example 1 was followed for the same alloy, Rene 100, in steps to prepare samples for rupture test, but at respective holding times at each temperature. The specimens remained for convenience at the time periods listed below in Table II, which were actually longer periods than necessary because of intervening overnight and weekend circumstances.

TABLE II Time in C "F hours Remarks l 170 2138 56 Just below incipient melting temperatures of lowest melting phase 1 I 2l56 Gradual increase to avoid incipient melting of phases as they solutionize i190 2174 6% l200 2l92 [5% Overnight for con- 12 IO 22 1 0 7% vemence l2l5 22l9 l l220 2228 64 Weekend for convenienee Cool to about:

212 ln inert atmosphere cooling chamber FIG. 5 illustrates a specimen of the Rene I00 alloy which has been heat treated according to the above example 2 and subsequently cooled in an atmosphere of argon to room temperature in less than 10 minutes. The structure apparent is 7' phase in a matrix of 7 phase. The y phase has precipitated out of the fully solutionized alloy during the cooling to room temperature. As compared to the as-cast alloy of FIG. 2, no low melting phases are present.

Of course, the holding times at the specific temperatures may be shorter than those listed in Table II. The minimum times at each temperature may be readily determined for the alloy being heat treated by examining a specimen periodically for incipient melting, to make sure that the temperature is not excessively increased before the segregated phases are put into solution sufficiently to increase the incipient melting point. The final temperature of the heat treatment of the Rene I00 alloy was l,220 C (2,228 F). At this point the segregated phases were substantially all solutionized.

It will be understood, of course, that for economic reasons the length of time of the heat treatment should be as short as possible, but long enough to allow diffusion to take place without incipient melting occurring.

Thus, the progressive heat treatment according to the invention, is continued from a beginning temperature just below the incipient melting range up to a final temperature below the melting point of the alloy, at a rate and for periods of time sufficientlylong at each temperature to completely diffuse the segregated elements and solutionize the low melting segregates into the balance of the alloy without incurring any incipient melting. This progressive heat treatment may be'performed either by continually increasing the temperatures gradually and continuously but at a predetermined controlled rate such that the phases of the segregates are substantially solutionized; or, as in Example 2, the workpiece may be progressively subjected to increasing temperatures in a series of steps, by holding the workpiece for a specific period of time at each temperature before increasing the temperature to the next level.

The mechanical properties of the Rene I00 alloy solutionized according to the present invention were compared with an as-cast specimen of the Rene 100 alloy. The results are as follows:

As-cast Rene 100 alloy Heat treated per Example 2 Hours to Rupture hrs. 591 hrs.

It is thus apparent that a substantial improvement in mechanical properties can be obtained in accordance with the heat treatment of the present invention.

It will be apparent that other alloys which tend to form phases segregated capable of being solutionized can be heat treated in a manner similar to that described herein.

It will be obvious to those skilled in the art, upon reading the foregoing disclosure, that many modifications and alterations in the method steps and in the specific compositions disclosed may be made within the general context of the invention, and that numerous modifications, alterations and additions may be made thereto within the true spirit and scope of the invention as set forth in the appended claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. A method of removing low melting segregated phases from a cast body of a nickel base superalloy which comprises the steps of:

a. subjecting the cast body to a heat treatment at a temperature below the incipient melting temperature of said phases and for a sufficient time whereby the incipient melting temperature of the resulting mass is increased,

b. gradually raising the temperature of the heat treatment such that the temperature is maintained below the increased incipient melting temperature until the low melting segregated phases form a substantially homogeneous mass with the matrix, and c. cooling said body to ambient temperature. 2. The method of claim 1, wherein said nickel base superalloy consists essentially of the following composition:

Weight Percent 9 S Chromium Cobalt l 5.0 Titanium 4.20 Aluminum 5 .50 Zirconium 0.06 Molybdenum 3 .00 Boron 0.015 Silicon 0.50 Carbon 0.] 8 Manganese 0.50 Iron 1.0 max Vanadium l .0 Nickel balance, 

2. The method of claim 1, wherein said nickel base superalloy consists essentially of the following composition: Weight Percent Chromium 9.5 Cobalt 15.0 Titanium 4.20 Aluminum 5.50 Zirconium 0.06 Molybdenum 3.00 Boron 0.015 Silicon 0.50 Carbon 0.18 Manganese 0.50 Iron 1.0 max Vanadium 1.0 Nickel balance, and the heat treatment comprises heating the cast body to a temperature of about 1,170* C. and then raising the temperature stepwise up to a maximum of 1,220* C.
 3. The method according to claim 1, wherein the temperature of the heat treatment is continuously and gradually increased.
 4. The method according to claim 1, wherein the temperature of the heat treatment is increased stepwise. 